Flight Control
I really like simple games, which you can play where ever you are - and which still has enough challenges to be interesting for quite a while. One of the best games I’ve played for a long time is Flight Control for the iPhone/iPod Touch. The game was developed by Firemint.
It’s a fairly simple game. You’re in control of an Airport and all you have to do is make sure the planes land at the runways. To do this you simply draw their designated paths on the screen. There are 4 different aircrafts - helicopters, propeller planes, small jets and large jets. As you play more planes concurrently enters the airspace - and eventually it gets quite busy and avoiding mid-air collisions a challenge.